Are you asking about the classic rolmphones? 120/240/400, or the 300/600 series, optisets, or what?
On the 120/240/400 classic phones, if you look at the bottom of the handset (the side that goes toward your face) you will see a "Rolm" logo. If the word "Rolm" has an "H" stamped after it the handset is hearing aid compatible.
I know all of the optisets and optipoints are - the only ones I'm not totally sure of are the 300/600 series.
If all else fails, fall back on this snipped I am quoting from a story in Nation's Business, May 1993:
[Begin quote]
The rule is the result of an interpretation of legislation passed by Congress in 1988; the measure required that telephones manufactured in or imported into the United States after Aug. 16, 1989, had to be hearing-aid compatible.
The 1988 law, the Hearing Aid Compatibility Act, also reiterated a 1982 Telecommunications for the Disabled Act requirement that all "essential" telephones be hearing-aid compatible. Essential phones were defined as coin-operated, those provided for "emergency use," and others "frequently needed for use" by people with hearing aids.
[End Quote]
So I guess, except for the classic series where you should look for the "H" on the handset, that you can assume everything else is.